A technology is known that, with a view to detecting the location of a radio frequency identification (RFID) tag, utilizes phases of plural tag signals that are obtained in accordance with a user moving an RFID tag-compatible reader and determines a bearing on which the RFID tag is disposed (for example, see Patent Document 1).
However, in the technology described above, although the bearing on which the RFID tag is disposed can be detected, a distance to the RFID tag is not detected. A technology is known (for example, see Patent Document 2) that detects a distance to an RFID tag as described below. A reader transmits plural signals to an RFID tag using plural signals with different fundamental frequencies. In response to these signal transmissions, the RFID tag applies backscatter modulation to the plural signals. The reader determines phases of the plural backscatter-modulated signals received from the RFID tag, and determines a rate of change of the phases of the backscatter modulation signals with respect to a rate of change of the fundamental frequencies of the transmitted signals. The reader then utilizes information on the determined rate of change of phase to calculate the distance to the RFID tag.                Patent Document 1: European Patent Application, Publication No. 2779020        Patent Document 2: U.S. Pat. No. 7,119,738        